Automatic atherectomy system

ABSTRACT

An automatic atherectomy system uses the rotary burr at the tip of a catheter as a sensing device, in order to measure both the electrical conductivity and permittivity of the surrounding tissue at multiple frequencies. From these parameters it is determined which tissue lies in the different directions around the tip. A servo system steers the catheter tip in the direction of the tissue to be removed. In non-atherectomy applications the rotary tip can be replaced with any desired tool and the system can be used to automatically steer the catheter to the desired position. The steering is done hydraulically, by pressurizing miniature bellows located near the catheter tip.

FIELD OF THE INVENTION

The invasion relates to medical devices and in particular to proceduresin which an undesired tissue has to be removed without harming anadjacent desired tissue, such as in atherectomy.

BACKGROUND OF THE INVENTION

In many medical procedures an undesirable tissue is adherent or touchinga desired tissue and the removal of the undesired tissue has to be donewith extreme caution in order not to harm the desired tissue. A wellknown example is atherectomy, the process of removing plaque from bloodvessels. The most common method of atherectomy is based on the use of ahigh speed rotary burr, mounted at the end of a very flexible catheter.The burr pulverized the plaque into such fine particles that they can beleft in the blood stream. A well known system is manufactured by theBoston Scientific company (www.bostonscientific.com) under the nameRotablator™. No further data is given here about this system as it is awell known commercial system. Other potential uses of the invention areremoval of tumors, such as prostate cancer, liposuction, dental work andmore. Today most these procedures are performed by a surgeon ismanipulating a surgical tool (directly or remotely) while observing thetool position using means such as fluoroscopy or ultrasound, or bytactile feel. In some procedures there is no need to remove tissue butthere is still a need to navigate within the body, such as directing acatheter through the blood system. The present invention can save themajority of the surgeon's time and operating room expenses

The present invention provides an automated way to navigate within thebody and remove undesired tissue without doing any harm to desiredtissue, even in situations that the undesired tissue is adherent. Thesame invention can be used for just navigation, without tissue removal.The preferred embodiment shown is atherectomy. In atherectomy there is aneed to differentiate between plaque and blood vessel wall. It is wellknown that plaque has different electrical properties than blood vesselwall; however the blood vessels are full of blood which has electricalproperties similar to the vessel wall. In order to automate atherectomya discriminator between vessel wall, plaque and blood is required. Also,it is desired to sense proximity to a vessel wall, not just contact. Thepresent invention provides exactly this capability. A similar situationexists in some tumor removal procedures: some tumors have differentelectrical properties than healthy tissue but the in-situ measurement ofthese properties is complicated by the fact that the voids left in theprocess of tissue removal are being filled with fluids which affect themeasurements. Prior at attempts to automate atherectomy relied on aguide wire (which can not be used in case of complete occlusion) or ondevices to help the surgical tool glide in the correct trajectory withinthe blood vessel. Since the plaque can be softer or harder than thevessel wall, it is very difficult to rely on such “self steering”methods. The current invention identifies the different materialssurrounding the rotary burr at the tip of the atherectomy catheter andautomatically steers the burr to remove the undesired tissue, such asplaque.

SUMMARY OF THE INVENTION

The invention uses the tip of a catheter as a sensing device, in orderto measure both the electrical conductivity and permittivity of thesurrounding tissue at multiple frequencies. From these parameters it isdetermined which tissue lies in the different directions. A servo systemsteers the catheter tip in the direction of the tissue to be removed. Innon-atherectomy applications the rotary tip can be replaced with anydesired tool and the system can be used to automatically steer thecatheter to the desired position. The steering is done hydraulically, bypressurizing miniature bellows located near the catheter tip.

In general, the invention can be used for a broad range of applicationsas the invention does not rely on the type of procedure used. It can beused with rotary burrs, stents, guide wires, suction, electro-surgeryetc.

In atherectomy there is a need to differentiate between at least threetypes of tissue: vessel wall, plaque and blood. Both vessel wall andblood have high conductivity and high permittivity, while plaque has lowconductivity and permittivity. The key for differentiating blood fromvessel wall is the change in permittivity with frequency: thepermittivity of the vessel wall falls much faster as the frequencyincreases.

Other features and advantages of the invention will become apparent bystudying the description of the preferred embodiment in conjunction withthe drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of the invention.

FIG. 2 is an isometric close-up view of the catheter tip.

FIG. 3 is a cross section of the catheter tip showing the steeringmethod.

FIG. 4 is a schematic diagram of the tissue discriminator.

FIG. 5 is a graph of the different waveforms produced by thediscriminator.

FIG. 6 is an isometric view of the actuation mechanism.

FIG. 7 is an isometric view of an alternate sensing method.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a blood vessel 1 having a wall 2 contains undesiredplaque 3 as well as blood 4. A. atherectomy tool 5 is introduced usingcatheter tube 6. The tool is driven by air motor 8 via flexible rotatingcable 7. No further details of the atherectomy system are given, asthese are well known commercial systems such as the Rotablator™ system.An electrical contact 9 measures the electrical impedance between cable7 and the return path which is ground (the patient is electricallygrounded). The discriminator 10 measures the complex impedance to groundby measuring the In-Phase current (I) and the Quadrature, or 90 deg outof phase current (Q). From these measurements the conductivity andpermittivity of the tissue can be computed, based on the well knownmethods of electrical impedance measurements. A full explanation isgiven later. Based on the measured value, the type of tissue isdetermined by computer 12 and the catheter is automatically steered byhydraulic actuator 13 (via tubes 14) to remove the undesired plaque 3.As it approaches the wall 2, the electrical properties start changingallowing precise and gentle steering and removal up to the wall 2 butwithout actually touching the wall. This is possible as the measuredproperties are also a function of tissue thickness, so when the plaquebecomes very thin the properties of the underlying layer are showingthrough. In order to determine the rotational orientation of burr 5 asense wire 11 is used in conjunction of a conductive strip on burr 5.

Referring now to FIG. 2, the rotary burr 5 has a standard diamond powdercoating 15 and is rotated at high speed by cable 7. It is made fromelectrically insulating material such as ceramic, with the exception ofmetallized strip 16. Alternatively, it can be made of metal and coatedwith a hard ceramic coating except for strip 16. A sense wire 11terminates with tip 21 very close to burr 5. Once per rotation strip 16comes close to tip 21. This point can be detected by measuring theelectrical impedance between wire 11 and cable 7. A sharp drop signifiesthis point, which is used as a rotational reference point. Catheter 6contains five channels on top of the central channel used by cable 7.Channel 18 is used for sense wire 11. Channels 17 are used to steer burr5 in the desired direction by inflating sections of bellows 20.Referring now to FIG. 3, bellows 20 is divided into four separatesections 19 connected to channels 17. Pressurizing a section will causeit to elongate, bending bellows 20 in the opposite direction. Fluid 22is a saline solution or pure water. For lower electrical noise theoutside of catheter 6 is metallized with a very thin coating 23. Asub-micron thickness, deposited by sputtering or evaporation, issufficient. Such a thin coat does not affect flexibility.

The discrimination of tissue types is shown in FIG. 4. To discriminateplaque from the wall of a blood vessel by electrical properties isrelatively easy and well known in the medical literature, as plaque hasa higher electrical impedance (both lower conductivity and lowerpermittivity). It is more difficult to differentiate the wall from theblood filling the vessel, as both have high conductivity and highpermittivity. However, the permittivity of the wall falls much faster(by about a factor of 1000 faster) with frequency. This can be seen fromthe following table. While the cited values differ between studies, allstudies show that permittivity of blood falls much slower thanpermittivity of the vessel wall as frequency increases.

log₁₀(Freq) 3 5 6 7 8 Conductivity (S/m) Blood 0.70 0.70 0.70 1.00 1.49Fat 0.025 0.025 0.030 0.040 0.060 Muscle 0.40 0.40 0.40 0.40 0.75Fibrous 0.24 0.24 0.24 0.29 0.33 Material Calcium 0.08 0.08 0.10 0.120.17 Vessel Wall 0.58 0.58 0.58 0.67 0.83 Relative Permittivity Blood4100 4000 2000 300 75 Fat 20000 100 50 30 12 Muscle 400000 10000 8000200 70 Fibrous 2000 500 50 5 3 Material Calcium 10500 500 250 70 30Vessel Wall 100000 5000 4000 100 30

The impedance of the tissue to ground (the patient is grounded) is shownschematically as impedance 24. A current is sent from oscillator 26 viaresistor 25, contact 7, cable 7 and burr 5 to the tissue impedance 24.The lower the impedance 24 the lower the voltage at contact 9 will be.Both the in-phase component I and the quadrature component Q is measuredbuy any one of the standard methods of AC impedance measurement. By theway of example, the I component is found by multiplying output f1 ofoscillator 26 with the voltage senses at contact 9 using an analogmultiplier 30. The Q component is found by multiplying the same voltagewith the output of f1 shifted by 90 degrees by phase shifter 29. Theoutput of the multipliers are filtered by capacitors 31 and converted todigital by A/D converters 38. This is the standard sine and cosineseparation method for finding the conductivity and permittivitycomponents of a complex impedance. For frequencies below a few MHz, thevoltage at contact 9 can be digitized and the derivation of theconductivity and permittivity can be done completely via digital signalprocessing. In order to generate the rotational reference pulse, thepoint when the voltage of sense wire 11 drops each revolution has to befound. The actual voltage can vary over a wide range, depending on thesurrounding tissue, but the dip is always when the conductive strip 16(see FIG. 2) is nearest to tip 21. By comparing the average voltage atwire 11 to the instantaneous voltage, the reference point is foundindependent of voltage. Signal f1 is fed to sense wire 11 via resistor32. The sine wave envelope is detected by diode 33 and capacitor 34. Theaverage is derived by resistor 35 and capacitor 36. Comparator 37generates a positive output when the instantaneous value is below theaverage value. Again, the reference pulse generation can also be digitalif the signal on wire 11 is digitized from the start.

Since the rate of change of the permittivity with frequency is required,at least two frequencies have to be used, three would be even moreaccurate. These are generated by oscillators 26,27 and 28. For eachfrequency the circuit shown has to be replicated. It is also possible touse a single variable frequency source and single detection circuit andmultiplex the detection process.

A typical discriminator output is shown in FIG. 5. Graph 43 is theamplitude of the sinewave at sense wire 11, used to generate therotational reference. Graph 39 shows the conductivity, derived from theI component (the higher the conductivity the lower the I component willbe). Graph 40 shows the permittivity, derived from the Q component (thehigher the permittivity the lower the Q component will be). Graph 41shows the permittivity at a much lower frequency. The horizontal scaleis in degrees relative to the reference pulse, which is created when tip21 is nearest to conductive strip 16. In this example tip 21 is drawnclose to the vessel wall. Using just the data at f1, it is difficult totell the wall (0-90 degree range) from blood (above and below burr 5, at90-180 degree range and 270-360 degree range). At the second frequencyf2, the permittivity in the wall area increases much faster than in theblood area. The plaque is easy to spot as it has much lower conductanceand permittivity. Using the values of table 1 for frequencies of 1 KHzand 10 MHz, the conductivity and permittivity of plaque(fat+calcium+fibrous material) are below 0.1 S/m and 50, while blood isabove 0.7 S/m at both frequencies and wall is above 0.58 at bothfrequencies. The permittivity of the wall is much higher than blood at 1KHz (100,000 vs. 4100) but falls much faster at 10 MHz, dropping afactor of 1000 for the wall but only a factor of 14 for blood. Thisexample shows that by using just three factors: conductivity,permittivity and ratio of permittivity at 10 MHz to 1 KHz the threetissues can be discriminated with a large margin. Adding a thirdfrequency f3 increases the accuracy. Note that the rotational speed ofthe burr 5 is about 1-3 KHz. For oscillator frequencies below that, theresults will have to be sampled and integrated over many rotations. Thisis not a problem, as the steering is done at a much lower bandwidth thanthe measuring. An alternative is use a frequency of about 100 KHz as thelowest oscillator frequency. A second alternative, shown in FIG. 7, isto replace the rotary tissue sensing by four sense wires 11 instead of asingle one, and have each one connect to a discriminator. Each one ofthe wires corresponds to one actuator direction. The advantages are:

1. A completely standard burr can be used, however sensing does notextend tip of burr. 2. System can be used for applications not requiringrotary burrs.

3. Only low frequency processing is required, as processing can be doneat the steering bandwidth instead of the rotation speed. Steeringbandwidth is below 100 Hz.

The catheter has four actuation channels 17 and four sense wires 11terminating in four tips 21. If a burr is used, tips can protrude topartially envelope burr.

The computer 12 in FIG. 1 performs the discrimination between tissuesbased on the rules shown above and steers the burr 5 into the undesiredtissue, in this case plaque. Clearly the decision rules and parameterswill change with the application and the tissue used. A large data baseof impedance data for a large number of tissues is posted on the ItalianNational Research Council website at:http://niremf.ifac.cnr.it/cgi-bin/tissprop/htmlclie/uniquery

There are similar data bases available on the internet for properties ofmalignant tumors versus healthy tissue.

The hydraulic actuators 13 are shown in FIG. 6 in conjunction withFIG. 1. A motor, such as a stepper motor, 48 is driven from computer 12via a standard interface. A piston 45 is moved in a cylinder 44 via theaction of a thread 46 and a mating female thread 47. The pressure istransmitted via hypodermic tubing 14 to channel 17 (not shown) incatheter 6. Four identical units are used for +X, −X, +Y and −Y.

By the way of example, burr 5 is a standard burr with an externaldiameter of between 1.5 to 2.5 mm. Because the system is automated asingle small burr can be used for all blood vessel sizes, as thecomputer will steer the bar in all radial directions to clean a largevessel. Catheter 6 is slightly smaller than burr 5. Diameter of piston45 is 1-2 mm and stroke is about 10 mm. Piston 45 and cylinder 44 aremade of very hard material, such as alumina, ruby or tungsten carbide,with a lapped fit. The pressure of the fluid is fairly high, typically50-100 Kg/cm2. Typical component values for the discriminator 10 are:frequencies in the 1 KHz to 1 GHz range, time constants of filter 31 ofabout 10-100 uS, time constant of capacitor 34 of 10-100 uS, timeconstant of capacitor 36 and resistor 35 of 10-100 mS.

An alternate way of steering is by using push-wires in channels insteadof a liquid. The actuators and catheter are very similar to the onesdiscussed earlier.

Still another way of steering is use catheter tips made of ferromagneticmaterial and have a controlled external magnetic field. A variation is asystem having a fixed external field and a catheter tip carrying threeorthogonal coils to generate a force in any desired direction. This isavailable as a commercial system under the trademark Niobe. It is soldby the Stereotaxis corporation (vwww.stereotaxis.com).

While the preferred embodiment relates to atherectomy and usedelectrical impedance sensing other applications and other sensingmethods are part of this invention. By the way of example, differenttissues can be discriminated by their mechanical properties such asstiffness, hardness and damping. This can be sensed by a vibrating tip.Tissues can also be discriminated by thermal properties. A tip similarto FIG. 7 can carry four temperature dependent resistors instead ofsensing tips 21. A constant current is passed through resistors andtheir temperature is measured by the voltage drop across them. Differenttissues have different heat conductivities: plaque will conduct lessthan blood vessel wall while blood will conduct heat rapidly, asconvection exists.

Also, the word “automatically” in this disclosure and claims should bebroadly interpreted, from a simple assist to the surgeon in operatingsurgical systems to fully unattended operation of such a system. In theminimal version the surgeon fully controls the system; the tissuediscriminator just assists the surgeon in the decision and operation ofthe atherectomy or other system. In a fully unattended operation thecatheter can also be automatically advanced into the body and can beprogrammed to enter the correct blood vessel when coming to a junctionpoint where there are multiple choices of routes. In the same manner,the “tool” or “catheter tip” should be broadly interpreted to includeboth contact tools (burrs, rotary wires, blades, suction,electro-surgery etc) as well as non contact tools (lasers, water-jet,gas jet etc).

1. A method of automatically controlling a tool for removing non-desiredtissue from the body without harming desired tissue, said methodcomprising the steps of: discriminating between desired and non desiredtissue based on sensing the properties of the tissue adjacent to thetool tip; automatically steering said tool in the direction of thenon-desired tissue; and eliminating the non-desired tissue.
 2. A methodfor automated removal of plaque from blood vessels by the use of arotary tool, said method comprising the steps of: discriminating betweenblood vessel walls, blood and plaque based on sensing the properties ofthe tissue adjacent to the tool tip; automatically steering said tool inthe direction of the plaque; and removing the plaque by rotary abrasion.3. A method of automatically steering a catheter in a body lumen, saidmethod comprising the steps of: discriminating between the wall of saidlumen and the inside of said lumen based on sensing the properties ofthe tissue adjacent to tip of said catheter; automatically steering tipto avoid damaging said wall.
 4. A method as in claim 1 wherein saiddiscrimination is based on electrical properties.
 5. A method as inclaim 2 wherein said discrimination is based on electrical properties.6. A method as in claim 3 wherein said discrimination is based onelectrical properties.
 7. A method as in claim 1 wherein saiddiscrimination is based on mechanical properties.
 8. A method as inclaim 2 wherein said discrimination is based on mechanical properties.9. A method as in claim 3 wherein said discrimination is based onmechanical properties.
 10. A method as in claim 1 wherein saiddiscrimination is based on thermal properties.
 11. A method as in claim1 wherein said discrimination is based on the electrical properties ofconductivity, permittivity and changes of permittivity with frequency.12. A method as in claim 2 wherein said discrimination is based on theelectrical properties of conductivity, permittivity and changes ofpermittivity with frequency.
 13. A method as in claim 1 wherein saidsteering is done hydraulically.
 14. A method as in claim 2 wherein saidsteering is done hydraulically.
 15. A method as in claim 3 wherein saidsteering is done hydraulically.
 16. A method as in claim 1 wherein saidsteering is done by using a magnetic field external to the body.
 17. Amethod as in claim 2 wherein said steering is done by using a magneticfield external to the body.
 18. A method as in claim 3 wherein saidsteering is done by using a magnetic field external to the body.
 19. Amethod as in claim 2 wherein said discrimination is based on the factthat plaque has lower electrical conductivity than blood or blood vesselwall, and the electrical permittivity of blood vessel walls falls fasterwith frequency than the permittivity of blood.
 20. A method as in claim2 wherein the rotary action of the tool is used to scan the differenttissues surrounding the tool.